For various reasons, such as expense, telephone operating companies often utilize nonloaded twisted wire pairs for the transmission of voice frequency (VF) signals. Beyond certain cable lengths it becomes necessary to insert repeaters in order to maintain adequate transmission levels at all frequencies of interest. Since the cable exhibits varying characteristic impedance with frequency while the repeaters usually have a constant in-and output impedance, it is necessary to convert the cable impedance to match that of the repeaters for proper operation of the total system.
U.S. Pat. No 3,814,867 issued June 4, 1974 to C. Wendell Boucher discloses an "Active Shunt Impedance For Compensating Impedance Of Transmission Line" suitable for the application at hand. However, the circuit taught therein exhibits instabilities in the form of undesired oscillations when connected for operation in telephone systems. Its utility for such applications is thus diminished. In a system where more than a single impedance matching device is necessary, spurious oscillations and instabilities, even though they may be outside the frequency band of operation, are not acceptable.